T helper cells play an important role in the regulation of the immune response to helminthic parasites. Understanding their function in infectious disease is therefore important for the development of therapies and vaccines that promote host protective immune responses. T cell costimulatory signaling by CD28/CTLA-4 interactions with B7 molecules is required for T helper effector cell function during the primary mucosal immune response to the murine nematode parasite, Heligmosomoides polygyrus. However, the individual roles of CD28 and CTLA-4 remain unclear with increasing evidence suggesting that together they may regulate the T dependent immune response, promoting T effector cell function early in the response and delivering negative signals that downregulate the immune response at later stages. In this proposal, we will specifically block CD28 or CTLA-4 interactions at different stages of the primary response to H. polygyrus, using blocking anti-CD28 or CTLA-4 antibodies, and also CD28-deficient mice, to examine differential effects of these molecules during the course of the immune response. We have also found that the primary H. polygyrus mucosal immune response is intact in CD28-deficient mice, whereas in a number of other systematic responses it is blocked. We will examine whether the CD28- independent nature of this response is characteristic of other parasites, in particular T. muris, and conversely will determine whether a systematic immune response to H. polygyrus is also CD28-independent. The systematic H. polygyrus immune response model will be further used to identify adjuvant-like constituents of the nematode parasite that might drive CD28-independent T cell differentiation to effector function. In addition, the role of IL-4 receptor signaling in driving CD28-independent T cells differentiation during the mucosal or systematic response will be investigated. Memory T cell induction as well as tolerance are also regulated during the primary response and we will examine whether blocking costimulatory molecule interactions during the primary response also influence the challenge host protective immune response to H. polygyrus, a model system that greatly facilitates studies of the memory response, since the primary response, associated with chronic infection, and the secondary response, associated with worm expulsion, can be easily distinguished. These studies should provide useful insights into the roles of CD28 and CTLA-4 in promoting or downregulating the immune response to nematode parasites.